In [1]}, a concrete proposal was made to specify the operators \(\mathcal {G}_A, \mathcal {G}_B, \mathcal {G}_C\)  above. It is done in the special case where the fact-sets all carry the same group structure \(G\) . More precisely, the group structure is given by bijective maps which send facts to group elements:
\(\mathcal {I}^A_B : F_{AB} \rightarrow G.\) 
